A new type of photosensitizers used in photodynamic therapy, which is based on photon upconverting nanoparticles, is reported. These photosensitizers are excitable with infrared irradiation, which has several times larger tissue penetration depth than the currently available ones. They are brought close to the target cancer cells through antigen-antibody interaction with good specificity and versatility. The design is also flexible in that various photosensitive molecules can be potentially adopted into the design. Results from in vitro experiments demonstrate their promise of becoming the next generation photodynamic therapy drugs.